1. Field of the Invention
The present invention relates to a process for preparing a can in which a seal is formed between a curl portion on the edge of the mouth of a can body and a flexible lid by heat-sealing, and also to a process for preparing a canned provision by using the so prepared can.
2. Description of the Prior Art
A sealing lid comprising a flexible substrate such as an aluminum foil and a layer of a hot-melt adhesive formed thereon is widely used for a bottle for packing a moisture-absorbing food or the like therein as an inner lid guaranteeing the integrity of the content (complete seal of the content) and providing an easily openable seal. However, industrial trials to use this sealing lid as an inner lid for a can for packing toasted layer or the like therein have proved unsuccessful. More specifically, in order to bond a sealing lid closely to the mouth of a can body, it is necessary to heat a hot-melt adhesive layer at a temperature higher than the softening point or melting point thereof, but since the thermal conductivity of the material of the can is much higher than the material of an ordinary bottle, the heat applied to the portion to be heat-sealed is conducted to other portions and it is very difficult to elevate the temperature of the portion to be heat-sealed to a level exceeding the softening or melting point of the hot-melt adhesive in a short time. Of course, the sealing portion of the lid can be heat-sealed to the can body by intensely heating the can body or the sealing portion of the lid for a long time. In this case, however, a lacquer or printing ink layer formed on the can body or the lid is scorched or discolored to degrade the appearance characteristics or the adhesive layer is thermally decomposed to cause deterioration of the adhesion and sealing characteristics.
These defects may be moderated to some extent if a hot-melt adhesive having a relatively low melting point, for example, an adhesive comprising an ethylene-vinyl acetate copolymer having a low melting point, a wax and a tackifying resin, is employed. However, the use of this hot-melt adhesive having a low melting point still involves problems. For example, the sealing layer of this low-melting-point hot-melt adhesive is brittle and it is readily peeled in fine pieces at the time of handing the sealing lid or opening the lid, and there is caused a so-called dusting phenomenon in which these fine pieces fall in the content or adhere to the content. Therefore, the use of this hot-melt adhesive is not preferred from the sanitary viewpoint. Moreover, a hot-melt adhesive composition of this type ordinarily has a peculiar smell and the flavor of the content is readily degraded. This is another defect.